Current wireless devices are able to access services via a number of different access technologies and networks. One may consider cost, quality of service (QoS), reliability, et cetera when selecting an access network to be used for an application level connection (application connection). For example, wireless local area network (WLAN) can be used for email downloads when such a known network is available, and otherwise a general packet radio service (GPRS) network or wideband code division multiple access (WCDMA) network can be used.
When taking a new access network into use, several protocol layers and functions need to be configured. For example, when taking a WLAN access network into use, WLAN authentication and QoS negotiation may have to be done, and an internet protocol (IP) layer may have to configure a new IP address, a domain name system (DNS) name, et cetera into use before application data transmission may be carried via the access network.
When utilizing available access networks dynamically, a handover may have to be performed when the used access network becomes unavailable or a more preferred access network becomes available.
Handover means moving a connection from one network to another one. There are two kinds of handovers. One is to move a connection within the same access network. This is the case for example when making cell reselections within one public land mobile network (PLMN). This is called a horizontal handover. Moving a connection between two different access networks, for example between WLAN networks operated by different operators or between a home WLAN and an operator WCDMA network, is called a vertical handover.
The purpose of a handover is to provide service continuity for users and applications. For example, moving a connection from a GPRS network to a WLAN network may offer the user a cheaper service when the WLAN network is available, and moving the connection back to the GPRS network again is to guarantee that a connection can be continued when the WLAN network becomes unavailable.
A general condition for a handover is that it should be seamless. This can mean, for example, that the handover should cause only little delay and it should not affect the application connection(s). In other words, the handover should occur without causing substantial delay or interruption in application level connection(s). Even when a connection is under handover the user should perceive no substantial changes in the connection.
However, in many cases during handover, application connections may suffer from delay, packet losses, et cetera. For example, a voice over IP (VoIP) application does not typically tolerate too long delays, or otherwise the voice quality suffers too much.
When performing a handover from an access network A to an access network B, the experienced delay depends on used access network configurations (for example, authentication and QoS negotiation used) and the access technologies of networks A and B. For example, when performing a handover from a WLAN network to a GPRS network having already active PDP contexts, application data transmission may typically continue immediately after handover. However, when performing a handover from a WLAN network to another WLAN network or between a WLAN network and a worldwide interoperability for microwave access (WiMAX) network, substantial delay (several seconds) may be introduced. This is mainly because the source and target access network may not be active at the same time. The new (target) access network needs to be configured before application data transmission may continue, and during the configuration application data transmission is suspended.
Traditionally, a fixed access network is used in most of the mobile devices. Some advanced phones may be able to select access networks based on network priorities and characteristics.
Current multi-radio mobile devices also have limitations affecting the handover procedure. For example, two WLAN networks can not be active for one device at the same time (this would require two Rx/Tx pairs, however even this might lead into interference (for example, when two WLAN networks operate at the same/closely located channel)). Also, WLAN and WiMAX networks can not be active at the same time, since there is too much interference, and/or same radio frequency (RF) components/pipes might be used for accessing both networks, and so on. All this means that when performing a vertical handover between such access networks, basically all operations including application data transmission has to be stopped in the access network currently in use, and then the new (target) access network has to be properly taken into use before data transmission can occur. During this time, which may be several seconds, application data transmission is suspended and data packets sent to the mobile device would normally be lost.
It is also possible that even if the source and target access networks could be active at the same time on radio level, a configuration problem may occur, for example, an IP layer configuration may fail for the target access network (for example, the target access network might block access to service). These kinds of problems are typically discovered only after taking the target access network into use, thus leading to increased delay and confusion from the application and user points of view.